Essential thrombocytosis (ET) is a myeloproliferative disorder resulting in an increased production of abnormal platelets. Reactive thrombocytosis (RT) is occasionally observed in clinical situations including chronic inflammation and malignancy. The aim of the present study was to evaluate the discriminatory efficiency of various laboratory tests in patients with ET and cancer-related RT. Forty-five patients with ET, 52 patients with RT, and 25 age-matched normal individuals comprised the study population. Plasma interleukin-1 alpha (IL-1a), IL-2, IL-6, tumor necrosis factor alpha (TNF-a), platelets, hematocrit, hemoglobin, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), lactate dehydrogenase (LDH) and ferritin were determined. We found increased levels of ferritin, LDH, CRP, ESR, IL-1a, and IL-6 in RT compared with ET (p < 0.01 to p < 0.0005). Hemoglobin, hematocrit, and platelets were significantly lower in RT than in ET (p < 0.0005). Furthermore, ferritin and ESR were negatively correlated with Hct, hemoglobin, and TNF-a, whereas ferritin was positively correlated with ESR, IL-1a, IL-6, and CRP, and IL-1a was positively correlated with IL-6. We consider that the aforementioned parameters should be included in the investigation of unexplained thrombocytosis for the differentiation of essential from cancer related thrombocytosis.